Can I connect a 5w 5w solar panel directly into a 10k mAh portable battery bank safely via USB 2.0, or does it need some sort of protection in-between like a solar charge controller or a schottky diode?
Example of battery bank (except one with usb 2.0 input): https://www.anker.com/ca/products/a1229
Example of solar panel: https://www.aliexpress.com/item/1005006012344670.html
Excellent. What does the monitor show for volts with full sun and no load?
To parallel them, you might get away with using an (very) old USB v1 hub. Or solder something up from the junk drawer or parts from an old junk pc